In thermal transfer printing successive sections of a donor sheet or web are fed through a linear printing region where they move, in contact with successive lines of a receiver, past a thermal print head comprising a linear array of selectively energizable, pixel-size heater elements. The print head or other means urge the juxtaposed donor and receiver sections into intimate contact at the print zone so that dye is transferred from the donor to the receiver in the pixels beneath energized heaters of the array. In multicolor thermal printing the receiver is moved through the printing zone a plurality of times so that a plurality of different color image components (e.g. cyan, magenta and yellow) can be successively printed on the donor, in register.
One common configuration for effecting such multicolor printing is described in U.S. Pat. No. 4,745,413, wherein the donor sheet is clamped onto the periphery of a print drum which rotates successive line portions past a linear thermal print array. A web bearing successive donor sections of yellow, magenta and cyan dye is fed through the print zone, between the print array and receiver, in a timed relation so that a different color donor section moves through the print zone with the receiver, respectively during each of the print drum rotations.
In typical prior art thermal transfer printers, the clamping apparatus provided on a print drum performs several functions. First, it must open precisely so that new receiver sheets can be reliably fed into engagable position and printed sheets can be reliably removed. Desirably it provides an accurate system for properly aligning the receiver sheet vis a vis the printing system. And, it must hold the sheet firmly in its aligned position during printing rotations of the drum past the print head.
In such clamping apparatus, different constructional approaches provide clamp actuator mechanisms on one end or on both ends of the print drum, which cooperate with abutment structures to open the sheet clamp via drum rotation. The simpler, less expensive approach is to provide actuators on one end only; however, this can result in an uneven width to the open clamp mouth (narrower at the end away from the actuator). The actuator-at-both-ends approach can eliminate the non-uniform gap opening but such construction are more costly from the parts viewpoint and require accurate alignments of the two end actuators during the print drum assembly.